Remember: a demon has a default mechanics presence in play which can always be relied upon. That presence moves like a human, can be perceived like a human (not "as," like), can hit like a human, can be hit like a human, can be communicated with, and so on. All of that applies no matter what else is described, and whatever narrational effects are necessary to make it consistent with the thing being (say) a lucky rock, well, that just means they are necessary.

A: None. All demons have the same "default mechanics presence" regardless of type.

I think I have a good imagination but I'm struggling to describe, in my head, an encounter between a person and a poltergeist which satisfies the mechanical presence and the narrational requirements of such an encounter. What does it mean to use your Stamina and (let's say) a shotgun to inflict damage, lasting penalties, even death, on something which doesn't have a body? How would you describe it? Would you invoke any "special case" bonuses or penalties, use different scores and how does that square with the default assumptions about demon/world interaction?

Same thing as happens in the movies. You shoot, there's a big bang, the demon manifests a big swirl or burst of activity, you stagger around, and it's gone. All is quiet. You must have driven it off. How, with a gun? Who knows? The impact on the viewer of the shotgun blast, possibly combined with the close-up on your desperate face, simply accounts for that. No in-story plausibility is necessary, and for some reason, no viewer ever questions it.

Or if you "miss," meaning the demon made its defensive roll, the shotgun blast has no effect. Why not? "Because you can't shoot a poltergeist, stupid!", completely overlooking the fact that the effect in the first paragraph either has or will have occurred in the same damn movie.

Use the rules absolutely as written. They utterly facilitate the above narrations; in fact, they exist for no other purpose.